Skeletons move without muscle or sinew. Zombies eat without hunger. Is it so strange that the dead can use a bow with no string?
Skeleton Bows are weapons that have been crafted by either necromancers or the hands of the dead themselves for their own use. The bow uses much the same animation magic as a skeleton does, replacing the sinew bowstring with necromantic energy. Any undead or necromantic inclined magic user can use the bow with no penalty. Greater undead, war aspected necromancers and those versed in the use of dead magic items can use the weapon with greater efficiency.
Our archers are greater than theirs, their arms do not tire, their strings do not snap, and shooting them full of arrows only replenishes their supply of arrows.
Skeleton Bows are made out specific types of wood, such as trees harvested from graveyards or mass graves, or the strange trees that grow in the underworlds and hells. A magic user can take this wood, after it is carved and shaped into a proper bow, and use a variant of the Animate Dead spell on it. This requires a gem of moderate quality to hold the enchantment, which is the weak point of the weapon. Smashing the gem destroys the bow.
Greater Skeleton Bows
These rare bows were made by greater undead, with more care to their construction, and attention to the materials used. Black diamonds, xerle or adamas, and demon jade stones are used, creating masterwork weapons capable of harming celestial beings and the immortal.